Well casing corrosion is a common and often serious problem having considerable economic significance. Accordingly, much effort is given to the detection, mitigation, and prevention of corrosion. In most cases, is it desirable to determine not only the presence or absence of corrosion, but also to estimate the rate at which corrosion is occurring. In the particular case of galvanic corrosion, caused by naturally-occurring electrochemical cells involving well casing metal and formation fluids, it is widely known that the metal loss rate in a given interval is directly related to the amplitude of electrical current leaving the well casing in that interval.
In conventional well logging practice, a logging tool consisting of two electrodes spaced apart by a distance on the order of twenty feet is lowered into the well. These electrodes are generally connected directly to surface instruments by a logging cable. A differential voltage will appear across these two electrodes, with a magnitude and polarity indicative of the amplitude and direction, respectively, of electrical current flow in the casing. By plotting the casing potential as a function of depth, it is possible to obtain a casing potential profile which a skilled interpreter may use to identify intervals in which corrosion is suspected. However, the differential voltages so measured are very small and are strongly influenced by multiple sources of error, and thus the voltage log is, in generally, not definitive. Furthermore, the electrical resistance of the interval is generally not known, with the result that the current (and hence metal loss rate) can only be inferred from the log.
Because in situ casing resistance is an important measurement in its own right, some logging tools are designed to measure both voltage and resistance. From these two measurements, casing current is readily obtained by Ohm's law. Resistance is ordinarily measured by a four-wire technique, where a known current is injected into the casing while measuring the corresponding voltage difference at two points between the current injection points. It will be appreciated that the low resistance of well casing, on the order of several tens of microohms per linear foot, makes accurate measurement of casing resistance a difficult proposition because of the small voltages produced by any reasonable current level.
It is known that, given instruments with suitable sensitivity and stability, very accurate measurement of corrosion rates, location, and extent are possible. However, the differential DC voltage potentials to be measured in either of the two above-noted logs (casing potential and resistance) are in the microvolt range. Accurate measurement of such low-level DC voltages is a classic and persistent problem in electrical science. Consequently, noise immunity benefits result from amplifying such microvolt potentials at the logging tool within the well, then transmitting the amplified signal up to the earth's surface for measurement and recording.
The amplifying of microvolt potentials within a well requires the use of very precise, low noise, stable amplifying circuits. A first class of amplifying circuits potentially applicable to this environment is conventionally known as chopper stabilized amplifiers. However, chopper stabilized amplifiers modulate input signals to completely remove all DC components, amplify signals using AC amplifying techniques, then demodulate the amplified signals to retrieve the original DC component. Unfortunately, chopper-stabilized amplifiers tend to be noisy, and the processes of modulation and demodulation each contribute their own error components. Using non-ideal, or "real world" components, AC amplifying techniques tend to be less precise than DC amplifying techniques of equivalent complexity under controlled conditions. Consequently, accuracy and stability suffer.
An example of a second class of such amplifying circuits is described in U.S. patent application Ser. No. 889,572, filed on Jul. 24, 1986, by Michael F. Gard, and entitled "Well Casing Potential Measurement Tool," which is incorporated herein by reference. Gard's amplifying circuit routes a low level differential DC voltage through a first switching device to a differential amplifier. The output of the differential amplifier couples to a first input of a second switching device and through an inverter to a second input of the second switching device. An output of the second switching device couples to a low-pass filter, and the low-pass filter output serves as an output from the amplifying circuit. This amplifying circuit converts only the offset voltage errors from the differential amplifier into an alternating voltage through synchronized switching of the first and second switching devices, then filters the amplified signal to remove that AC offset error voltage. The Gard circuit improves upon conventional chopper stabilized amplifiers because the DC signal is maintained as a DC signal and is not modulated into an AC signal then demodulated back into a DC signal.
Gard's amplifying circuit removes a large portion of offset voltage error from the total sum of offset errors that can accumulate in the amplifier circuit. However, it does not remove all offset errors which are introduced by the inverter and the low-pass filter. Therefore, even with the use of very high quality and expensive components in the inverter and low-pass filter functions, some error can remain in the signal output from the Gard amplifier circuit.